The generation of gas produced from the anaerobic decomposition of organic matter is a natural process occurring all around the world.
In recent decades the organic matter generated as by-product of human activities has been stored in large cells usually confined with under and overlaying membranes. The overlaying membrane could be described as daily cover and final cover.
The organic matter thus trapped is slowly drying since no more rainfall precipitations can penetrate the impermeable body of organic matter. Even during the drying period, the organic matter can produce gas as a by-product of anaerobic decomposition, but to a limited extend and for a limited period of time, because such decomposition requires water to occur.
The microbiological cycle of gas production requires as high moisture and temperature as possible to activate and accelerate the microbiological decomposition of the organic matter under anaerobic conditions.
Before this discovery, it was found that the artificial introduction of water into the biomass of a landfill, even trough it includes impermeable membranes at the bottom and at the top, improves the amount and duration of gas production from organic matter. This was generally called bioreactor landfill system.
However, the presence of standstill water below the surface level of the biomass of a landfill generally prevents the efficient capture of landfill gas using vertical wells. Therefore, the efficient extraction of liquid present in the biomass and of gas generated from the decomposition of the organic matter using conventional methods is more difficult.
It has also been observed that the internal pressure profile varies in the landfill following cycles of high and low positive pressure generating a pressure wave over time which has varying amplitude and a varying frequency.
The pressure wave frequency and amplitude varies according to internal properties of the body of waste such as waste type, waste density, waste moisture content, waste porosity, waste layering, and the waste age. Because these properties of waste layers and composition are not constant within the body of the landfill it creates anisotropic and heterogeneous conditions. These inherent conditions coupled with landfill internal variation of temperature, moisture content, organic matter content and waste distance from the atmosphere cause the pressure wave amplitude and frequency, at a given point, to be unsteady over time. This means that any pressure wave amplitude and frequency is difficult to predict.
However, observations are showing that:
Pressure wave formation is essentially caused by a sequence of pressure build-up in the landfill due to organic matter decomposition followed by pressure release towards the atmosphere (the zone of lowest positive pressure) through micro and/or macropores creating pathways to ensure that the landfill gas escape towards the atmosphere, whether located upwards or sideways.
Younger waste with a combination of high temperature, high organic matter content, high moisture content under a predetermined waste density, depth and porosity will have a high pressure wave frequency (see FIG. 1).
Older waste with a combination of low temperature, low organic matter content, low moisture content under a predetermined waste density, depth and porosity will have a low pressure wave frequency (see FIG. 2).
Older waste may also have lower amplitude than the amplitude of younger waste because of lower pressure built-up caused by preferential path created over time within the network of micro and macro pores towards the atmosphere (see FIG. 3).
Also, at a predetermined waste density, depth and porosity, the absence or depletion of moisture, temperature, organic content or a combination thereof will create low frequency/low amplitude pressure wave and indicate a reduction in landfill gas flow rate potential.